In the book "A Generous Orthodoxy", Brian D. McLaren describes himself as: "...a missional, evangelical, post/protestant,
liberal/conservative, mystical/poetic, biblical,
Anabaptist/Anglican, Methodist, catholic, green, incarnational,
depressed- yet hopeful, emergent, unfinished Christian". His book is a fantastic read, because in it he describes a tour throughout the various denominations and what he learned from actively participating, without prejudice, in each.
This wasn't "pick and choose" or "salad bar" Christianity - more along the lines of the blind bats flying by an elephant and perceiving it based on the part of the elephant each landed on. If you think about it, even the source material itself presents different viewpoints: we get a more complete picture of Jesus from the four Gospel writers, each of whom perceive Him in a different way. Mark portrays Jesus as a revolutionary whose movement initially failed. Matthew's gospel points to rabbi, a teacher, the expected Messiah. Luke portrays him as a compassionate hero who dies for all of humanity, and John's Jesus was a mystic. Who was right? Yes.
And Christianity's divergent evolutionary paths mean that in different churches, different lenses have evolved to focus on what whatever that church's founder thought was important. For example, Martin Luther and Calvin thought the crucifixion was a penal substitution, whereas the anabaptists thought that left no room for divine pardon, and the Catholics didn't like what that meant in terms of sanctification by the Holy Spirit. Even the styles of worship are radically different, from the pomp and circumstance of a full blown high church Episcopal mass, to a quiet gathering of Quakers in an unadorned room. Whether the Eucharist is seen as transubstantiation and an incredibly important sacrament, or a ritual of remembrance with deeply spiritual overtones, or not celebrated at all.
But in the latter 20th and early 21st centuries, we're seeing a strange juxtaposition of forces. One force is seeing church attendance decline and people in the pulpit despairing over increasingly secular influences and a greying of the congregation. The other is the rise of fundamentalism as people desperately seek answers. And in the same way that cell phones are displacing pay phones even though pay phones still exist, churches still technically exist, but people are seeking out spirituality and religion in other ways. Biker tabernacles, online churches, home churches, prayer meetings, new school evangelical churches that resemble coffee houses more than houses of worship, books in the stores on Kabbalah and Feng Shui. "I'm spiritual, but not religious".
And there's something rotten in the state of Denmark. The Episcopal Church watched entire congregations up and walk: either to the Catholic Church, or to another, breakaway and not officially recognized Anglican group, ACNA - over the ordination of an openly gay Bishop. Likewise, the Southern Baptist Church are in the midst of purging themselves of liberal elements, watching Pentecostal and charismatic movements gaining strength in the American as well as the global south.
In the world of any kind of engineering, sometimes you look at a structure, or you look at software code, or whatever it is you're building, and evaluate whether you can save what you have or do a re-build. Buildings are rebuilt with the same materials, programmers start afresh but carefully copy over source code from the relics of the last project, picking and choosing what fits and what doesn't. Or, in the words of Marcus Borg - "a way of seeing the Bible (and the Christian tradition as a whole) as historical, metaphorical, and sacramental, and a way of seeing the Christian life as relational and transformational."
In practice, people like McLaren visited various churches and took what they liked (not theological picking and choosing, but aspects of their worship). For example, whereas someone might have been put off by Pentecostals rolling around and speaking in tongues - they were participatory and actively open to God working through themselves, as opposed to standing there halfheartedly singing the same old hymns. And though there are different general ideas about emergent Christianity, for example the theological conservative trying to make church relevant, or the passionate advocate of making the church a transformational force, either personally or socially, or someone way out there like Spong, arguing completely for a post-theist understanding of the Bible - there are certain themes that are common to most.
One is Christian praxis - a term taken from liberation theology. The gist of it is to see things historically: Jesus was a social reformer who fed the poor and healed the sick, and transformed the lives of who He was around, and as a result - Christians have an obligation to make their own lives transformational, not only of self but the society in which they live. And given the way the first followers of the movement lived in koinonia, they see an opportunity to achieve this by moving closer to first principles.
A second is mission and evangelism from a post-modern perspective. Whereas old style evangelism was sometimes along the lines of inviting someone to read their Bible and be "saved" and/or outright scaring the pants off them with fire and brimstone dire warnings of Hell, they're trying to share the good news as they know it, and invite people to join in in the struggle to better themselves and others. Likewise, to liberate mission from its colonialist or tourist trappings and make whatever they're doing relevant.
And that also ties in with a more modern removal of hierarchy. In the same way that modern work emphasises collaboration over command and control, and the org chart's been flattened considerably at many work places, the emergents generally hope that good will rise up the ranks, that ideas will come from parishioners/community members looking for ways to transform themselves, understanding, worship and mission - as opposed to being dictated from on high. Emphasis is on personal testimony, shared study, shared meals (the original way the Eucharist was celebrated. Christianity at its inception didn't have priests, it had bishops and it had deacons. It didn't waste money on giant buildings, or maintain a clergy. Jesus said "wherever two or three of you are gathered". After all, Jesus pointed out that to be clergy was to serve, not to be served, and the most amongst you will be the least, and the least, the most. They're trying to live that.
This is happening within and without churches. Never mind the creation of entirely new types of churches - the old hierarchies are responding. The Episcopal Church looked at the 21st century as a challenge and change, and published the "Transforming..." series of books on how to apply some of this thinking to their tradition. And a high ranking Catholic bishop was arrested at Occupy Wall Street. The new Pope washed the feet of a Muslim female prisoner, and takes the bus to work, eschewing his predecessor's notorious Prada pumps.
A lot of theology has come about from iterations over translations of a Bible, and a lot of it is getting undone as people study the history and the languages. "Salvation" in the Bible talks more about transformation on earth than it does some kind of after-life levelling up. "Mercy" has moved far from its original meaning of "compassion". The word "righteousness" really translates to what we understand as "justice". And within certain parameters, (and here your mileage may vary) - parishioners are free as a self-managing group to find new, creative ways to express that spirituality and advance those principles, seeing themselves as a fraternity seeking improvement, rather than an institution of people coming to obey.